In order to explain “turn down” you have to first understand turnt or turnt up. That’s slang and a way for people to say that they’re having a really good time, sometimes with the help of alcohol or weed.

If being turnt up is so fun, why would someone want to turn (turnt) down?
That’s the point.

“Turn down for what” is a song about keeping the partying going. “Fire up that loud/Another round of shots” is Lil Jon telling you or someone to keep the party going. (Loud is slang for good quality marijuana.)

“Turn down for what?” is a rhetorical question. There’s nothing, at this moment in time, that’s worth coming down from this high, Lil Jon feels. What would make you want to sober up instead of being drunk/high/having a good time?